Color stabilization of acrylonitrile



Patentecl Apr. 27, 1954 2,6'76,97 7 I COLOR. STABILIZATION OFYACRYLONITBILE Erwin L. Carpenter, Stamford,onn.,. assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application March 31,1952,

Serial No. 279,680

3 Claims. (01. 260-465;?)

This invention relates to acrylonitrile. More particularly, it relates to an improved method of producing acryonitrile. Still more particularly, it relates to an improved method of purifyin crude acrylonitrile' to produce a substantially color stable product and to the product so obtained.

,Crude acrylonitrile is contaminated with traces of reactants and organic by-products. These will vary with the particular reaction employedfor its synthesis. In addition, the crude product is usually saturated with water, also a byproduct. To meet marketable standards of color, chemical and physical properties and purity, it is necessary to purify the acrylonitrile and remove therefrom substantially all traces of contaminating lay-products. As a practical procedure this is usually accomplished by distillation. The resulting acryionitrile is a water-white, mobile liquid, substantiall free of foreign organic substances and containing less than about 0.2% by weight of water.

Prior to its use in the manufacture of various end products, by necessity it is often stored for extended periods. Unfortunately, however, even in the absence of light, acrylonitrile is found to discolor badly during storage, often to the point where it is no longer suitable for its intended use. Different batches ofaorylonitrile may develop color at different rates, but it has been found a general fault of the commercial products. Since discolored acrylonitrile is useless in the manufactureof many products, particularly of synthetic fibers, this has been a troublesome problem.

Although the reason for color formation in commercial acrylonitri-le is not fully understood.

innumerous efforts have been made to devise a method to counteract it. Such a method, to bepractical, must necessarily meet certain requirements. It must promote color stability in sulficient degree for an adequately extended period. In addition, achievement of the result must be obtained While maintaining necessary standards of purity required of the product for subsequent use. Impure acrylon-itrile is as objectionable as discoloration;

For lack of any method capable of fulfillin these requirements, there continues to remain a definite demand for a suitable method of producing a color-stable acrylonitrile.

Accordingly, it is the principal object of this invention to provide a method for obtaining a color-stable acrylonitril'e. Such a method must overcome the failures of previous attempts at color stabilization. In addition, it should require no costly addition agents and should be simple in operation.

These objects have been met by a surprisingly simple method which is completely effective. It is'based on the surprising discovery that a correct water content, in the substantial absence of other organic impurities, will stabilize acryloni't r'ile against color change. In general, the method of the present invention consists in first treating crude acrylonitrile, as in the past, to substantially remove all traces of residual reactants and organic icy-product's, including water. To the resulting purified but dehydrated acrylonitrllethere is added a uniformly disseminated controlled amount of water. This simple but carefully clone step provides a color-stabilizing effect lasting overlong periods. The effectiveness of water for this purpose'is particularly surprising" in view of the-fact that water in the crude product has no such effect and was considered a necessary evil.

Removal of contaminants from crude acrylonitril is a problem of distillation. Although it is now well known how to separate organic contaminants from acrylonitrile by azeotropic distillation, it requires close control. This is because of the small difference in boiling points of the water azeotrope of the organic contaminants and the Water azeotrope of acrylonitrile. For practical purposes, therefore, the crude product is distilled, with substantially total reflux of the nitrile; until thecrude product is substantially dehydrated. Because of the greater difierence in boiling points of the organic contaminants and acrylonitrile, as comparedto their corresponding water azeotropes, apure acrylonitrile is then easily obtainedby'distillation.

To this purified but dehydrated product, water is added to achieve the necessary color stability. Exactly Why water prevents color formation is not understood. However, the factor is definite. Less thanabout 0.3% by weight Will not promote stability. The effectiveness of Water addition increases rapidly up to about lit-1.5% by weight after which the effect gained by added increments decreases Above Zed-2.5 the slight increase in effect gained by added increments becomes unimportant. Moreover, about 3% is approaching the saturation point of water in acrylonitrile at normal storage temperatures and, in addition, approaches a value at which the water content becomes undesirable for other reasons. Although the water content within the range of the present process, i. e., about 0.3% to about 3.0%, does not affect subsequent use of acrylonitrile for most purposes, the minimum content providing adequate stability is usually desired. A preferred range, therefore, is about 0.5% to about 1.5 by weight of water.

Purity of water employed for stabilization is an important consideration. Many sources of water contain various objectionable inorganic and organic impurities, the presence of which, in acrylonitrile in sufficient content, destroy its usefulness in the manufacture of many products. Often the content of these objectionable impurities in water is sufiiciently high to immediately contaminate the acrylonitrile. Even when initially present in water in less than contaminating amounts, accumulation of the impurities in the equipment over an extended period will eventually result in contamination of the acrylonitrile. The source of water, therefore, is of definite importance in the practice of the invention and care must be used in selection of a suitable source. Water obtained from distillation of crude acrylonitrile is substantially free of impurities generally present in municipal or industrial Water sources. Provided it is sufficiently free of by-products of acrylonitrile synthesis, it serves adequately for use in stabilization. It is definitely preferable, however, to use distilled Water, thus avoiding contaminating the acrylonitrile with any of the impurities discussed. In order to obtain the desired stabilizing effect, water employed for the purpose must be thoroughly diffused throughout the acrylonitrile mass. This may be done in any suitable manner. Preferably, it is done by pumping acrylonitrile from one tank to another with introduction of water at the pump. In this way thorough distribution of water in acrylonitrile occurs through the mixing action of the pump. The sam efiect may be obtained using a single tank associated with a pump and recycling conduit. Any other agitating arrangement may be employed provided it insures thorough diffusing of the water in acrylonitrile.

Once the discovery of this invention was made a series of tests were conducted to show the color stabilizing effect obtained by adding water to freshly distilled dehydrated acrylonitrile. These tests were made using samples of freshly distilled dehydrated acrylonitrile and like samples to which had been added varying amounts of water. For further comparison, a sample of crude acrylonitrile as noted above, normally substantially saturated with water, and a similar but dehydrated sample were tested. All samples were stored in glass containers in diffuse dayli ht at room temperature. At the beginning of storage and at varying subsequent intervals, each sample was examined for color by means of a spectrophotometer.

Color readings of the samples were obtained by comparison with readings made on a series of platinum-cobalt standard solutions utilized in the sanitary examination of water and approved by the American Public Health Association (Standard Methods for the Examination of Water and Sewage, 8th ed., 1936). Results ap- 4 pear in the following tables. In recording the color readings a number under 20 is considered water-white, 50 is very pale yellow, 100 is pale yellow, and 200 is yellow.

Table I Original Color Reading After Storage Acrylonitrile, Sample #1 Color Reading 2 Months 4 Months 6 Months Containing less than 0.3% Water. 10 107 240 370 Containing 0.5% Watcr 10 29 45 65 I Containing 1.0% Water..- 10 l4 19 28 Table II Color Reading Original After Storage Acrylonitrile, Sample #2 Color Reading 3 Months 6 Months Containing less than 0.3% Water. 15 188 370 Containing 0.3% Water 15 108 175 Containing 0.5% Water 15 48 100 Table III Color Reading Original Alter Storage Acrylonitrile, Sample #3 Color Reading 4 Month 7 Months Containing less thm 0.3% waten... 10 178 195 Containing 0.5% Water 10 20 41 Containing 1.0% Water 10 ll 19 Table IV Color Reading Original After Storage Acrylonitrile, Sample #4 Color Reading 1 Months 4 Months Containing less than 0.3% Water 13 90 190 Containing 0.5% Water 13 45 64 Containing 3.0% Water l3 7 8 Table V Color Reading Original After Storage Acrylonitrilc, Sample #5 Color Reading 3 Months 6 Months Crude containing less than 0.3%

Water 15 131 295 Crudc containing 3% Water... 15 110 272 These results clearly show that the mere presence of water, as in the undistilled synthesis product of Table V, does not promote color stability over that found in a similar, but dehydrated, product, also as in Table V. Tests in Tables I-IV show that color stability is not a characteristic of substantially purified, dehydrated acrylonitrile. However, by addition of small quantities of Water to the samples of Tables I IV, color stability is markedly improved. A comparison of Table IV With Tables I-III indicates that, in many instances, the small increase in color stability gained by the addition of more my copending application for United States Letters Patent Serial No. 177,780, filed August 4,

5 1950, entitled Color Stabilization of Acrylonitrile, now abandoned.

I claim:

1. In purifying crude hydrated acrylonitrile to obtain a commercially purified product wherein crude acrylonitrile is azeotropically distilled to remove substantially all the Water, and then further distilled to separate residual organic contaminants, the improved method of color-stabilizing the distilled acrylonitrile which comprises adding thereto sufficient water to increase the moisture content thereof to from about 0.3% to about 3.0% by Weight and thoroughly diffusing the water content through the resultant fluid mass.

5 distilled water is added to the distilled acrylonitrile in amount sufficient to increase any moisture content therein to about 05-15% by weight.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,388,507 Teter et a1. Nov. 6, 1945 2,452,554 Davis et al. Nov. 2, 1948 2,455,689 Marple et a1. Dec. 7, 1948 

1. IN PURIFYING CRUDE HYDRATED ACRYLONITRILE TO OBTAIN A COMMERCIALLY PURIFIED PRODUCT WHEREIN CRUDE ACRYLONITRILE IS AZEOTROPICALLY DISTILLED TO REMOVE SUBSTANTIALLY ALL THE WATER, AND THEN FURTHER DISTILLED TO SEPARATE RESIDUAL ORGANIC CONTAMINANTS, THE IMPROVED METHOD OF COLOR-STABILIZING THE DISTILLED ACRYLONITRILE WHICH COMPRISES ADDING THERETO SUFFICIENT WATER TO INCREASE THE MOISTURE CONTENT THEREOF TO FROM ABOUT 0.3% TO ABOUT 3.0% BY WEIGHT AND THOROUGHLY DIFFUSING THE WATER CONTENT THROUGH THE RESULTANT FLUID MASS. 